TRS Group, Inc. (TRS), as a subcontractor to AECOM, completed a Standard Fixed Price Remediation (SFPR) of trichloroethene (TCE) in groundwater in Western Connecticut using Electrical Resistance Heating (ERH). The objective of the ERH project was to achieve the prescribed remediation goal for dissolved TCE in groundwater within the treatment volume. The system incorporated 89 electrodes with co-located vapor recovery (VR) wells to treat a soil volume of approximately 30,900 cubic yards (yd3). Active subsurface heating extended from approximately 12 feet below grade surface (ft bgs) to the competent bedrock surface where the depth to bedrock within the treatment volume ranged from 22 feet to 45 ft bgs. AECOM is implementing bioremediation cross-gradient and downgradient of the source area for dissolved plume remediation.

Average concentrations of TCE in groundwater decreased on average 95.9% in the treatment volume following the application of ERH remediation. Groundwater TCE concentrations were reviewed based on four depth zones (shallow, mid, weathered bedrock and bedrock) based on the screened interval of monitoring wells in the treatment volume. Post ERH confirmatory groundwater sampling indicated that average concentrations of TCE in the four depth zones were well below the established remediation goal of 760 parts per billion (ppb). Analyses of temperature trends in the final ERH treatment volume, groundwater analytical results from samples taken outside the treatment volume, and general site information indicated that concentrations within the ERH focus volume had reached the point of diminishing returns. Soil vapor and steam capture was complete throughout ERH system operations and site, staff and public safety were monitored and maintained throughout the project.

Background

The wedge-shaped treatment area was located at a former metals processing and manufacturing facility in a mixed industrial and residential area and covers approximately 30,400 square feet (0.6 acres), as depicted in Figure 1.

The site was developed as a manufacturing facility in 1957 by the property owner. Operations included the use of chemical solvents with chlorinated volatile organic compounds (CVOCs) including TCE. Following a fire in 1994, the building at the site was razed in 1996.

The ERH treatment area was located in the vicinity of the former manufacturing area and former solvent storage areas. Prior to the ERH remediation, the highest reported TCE concentration within the treatment volume was 834,000 ppb. Following PVC monitoring well replacement in 2008, the highest concentration of TCE in groundwater was 46,000 ppb while the average was 5,583 ppb. The remediation goal for the site was to reduce TCE in groundwater to 760 ppb or less at 22 monitoring wells (MWs) in the treatment volume. Cleanup goals for soil focused on TCE in groundwater and were developed by AECOM as maximum concentrations to achieve applicable Connecticut groundwater standards for the closest environmental receptor; a small stream located approximately 400 feet north of the ERH treatment area.

The site soil lithology in the vadose and saturated zones is described as an overburden fill and/or urban fill material underlain by glacial till. The deeper saturated zone is weathered bedrock that is six inches to three feet thick and separates the overburden from the competent bedrock below. Groundwater at the site was reported to range from 5 to 14 ft bgs.

System Construction/Operations

The ERH system was designed and installed to remediate the TCE source area at the site. The ERH system consisted of 89 vertical electrodes with co-located vapor recovery wells to treat a soil volume of approximately 30,900 yd3. The electrodes were completed to variable depth intervals and installed to the surface of the competent bedrock. The top of the conductive interval started at 12 ft bgs. Subsurface temperatures were measured at eight temperature monitoring points (TMPs) with thermocouples spaced at 5-foot depth intervals through the heated volume. The ERH system consisted of a 2000 kW power control unit (PCU), a condenser/cooling tower unit, a 40 horsepower blower for vapor recovery, and vapor treatment consisting of a thermal oxidizer and vapor-phase granular activated carbon vessels (VGAC) as shown below in Figure 2. The thermal oxidizer was used during the initial part of the project when influent CVOC vapor concentrations were high and as CVOC vapor concentrations declined, vapor treatment was accomplished using two 2000 pound VGAC vessels plumbed in series.

Results

The ERH system operated across the entire treatment area for a period of 23 weeks. A total of 595 pounds of CVOCs were recovered in the vapor phase from the treatment volume during ERH. Groundwater was sampled on a monthly basis before, during, and following ERH treatment, from monitoring wells located within and outside of the treatment volume. Monitoring wells were installed at a variety of depths with screened intervals ranging from 5-15 to 37-42 ft bgs. Refer to the list below for details regarding the well screens and depth of monitoring wells in the treatment volume.

Average concentrations of TCE decreased over an average of 95.9% in the treatment volume following the application of ERH. Soil vapor and steam capture was complete and compliance with vapor effluent discharge parameters established for the site was maintained throughout ERH system operations. Refer to Figure 3 for details regarding groundwater TCE concentrations in the treatment volume.

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